In modern automobiles, radiators are designed so that access into the radiators becomes quite difficult, and intentionally so. In the ordinary operating and maintenance of an automobile, it is intended that the radiator shall remain sealed, and that if liquid needs to be added to the radiator, it is added by applying liquid into an overflow container. The radiator is designed to operate under a positive pressure of 10 to 20 psi over atmospheric, and the radiator and cooling system is intended to be tightly sealed most of the time.
As a result, the radiator caps are designed so that they are difficult to turn, and furthermore, during turning of the radiator cap to obtain access into the interior of the radiator, the cap must be depressed with downward pressure against a spring of substantial strength. Of course, all of this is a safety precaution so that the radiator cap will not be accidentally or casually removed for there is considerable danger in suddenly opening or removing the cap from a hot radiator, due to flying water particles and steam. Whereas radiator caps previously had radially extending lugs which could be gripped easily, most radiator caps nowadays are perfectly round.
Oftentimes access to a radiator cap is limited by other parts and accessories closely adjacent.